Mycelial Growth, Sporulation, and Virulence to Apple Fruit of AIternaria alternata Isolates Resistant to Iprodione. ALAN R. BIGGS, Associate Professor, University Experiment Farm, P.O. Box 609, Kearneysville, WV 25430. Plant Dis. 78:732-735. Accepted for publication 13 April 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0732.

Three isolates of Alternaria alternata from apple fruit were examined in the laboratory for sensitivity to iprodione; their EC50 values were 2.9, 2.4, and 2.7 µg/ml of iprodione. After 12 days on medium amended with iprodione at 25 or 250 µg/ml, resistant sectors occurred and six resistant isolates were selected and used for further study. Only resistant isolates grew on medium amended with 25 or 250 ?g/ml of iprodione. Sporulation in culture varied with isolate and source; isolate WV/1 and its resistant selections produced the most conidia. The amount of fruit rot caused by resistant isolates was similar to that caused by the sensitive isolates when inoculated into nontreated fruit. Dips of apples in iprodione prior to inoculations reduced lesion development of sensitive isolates but had no effect on lesion development of resistant isolates. In this study, the ranges of fitness and virulence of resistant isolates overlapped those of sensitive isolates, and resistant isolates exhibited no discernible differences in cultural characteristics or virulence from the sensitive isolates .